The present invention pertains to the field of control systems for controlling condensing head pressure in a refrigeration system which includes heat reclaim coils.
It is well know that in the case of refrigeration systems having condensers exposed to cold outdoor temperatures, the condensing temperature may fall too low for proper system operation, unless steps are taken to maintain condensing head pressure at a desired level. Many techniques for head pressure control are known in the art, including the standard techniques of shutting off condenser fans and partially flooding the condenser by means of a bypass and a control valve. In so-called bypass systems, a control valve, usually at the outlet of the condenser is connected to receive refrigerant from the condenser or from a bypass path, and the control valve operates the system, usually by comparing the condensing pressure to a reference pressure and controlling condenser flooding accordingly. In most prior art systems a fixed reference pressure is used, but a more efficient and advantageous method of controlling head pressure is set forth in our co-pending patent application Ser. No. 759,129, entitled "Refrigeration System Subcooling Control", filed Jan. 13, 1977, now U.S. Pat. No. 4,136,528 and assigned to the assignee of the present application. In that patent application we set forth the technique of raising head pressure to only the minimum pressure required to ensure subcooling and prevent the formation of flash gas at the critical point along the liquid line between the condenser and the thermostatic expansion valve for the evaporator. Sensors are provided for monitoring both subcooling in the liquid line at the entrance to the thermostatic expansion valve or other critical point in the line, and for monitoring a minimum of pressure differential across the expansion valve to ensure proper operation thereof. Appropriate signals from the two sensors are OR'd together and the result used to control the valve which controls flooding of the condenser. This has the advantage of keeping the head pressure high enough (during cold outdoor temperature weather) to maintain operation of the thermostatic expansion valves so that refrigeration capacity is unimpaired, while preventing inefficiencies which would be associated with compressing the refrigerant to any higher pressure.
In recent years the rising cost of energy and increasing emphasis on efficiency have promoted considerable interest in reclaiming the heat rejected by the condenser of a refrigeration system in a manner that will permit its use for heating the comfort conditioned air space in a building. This is particularly true in installations involving multiple refrigeration systems, for example in a supermarket. Typically, a supermarket has a great number of refrigeration devices including walk-in storage areas, open display cases and glass or solid door reach-in refrigeration units. It is common to use multiple systems wherein a number of subsystems may be utilized, each having its own compression, condensation and distribution elements, and each running a number of evaporator loads. During cold weather, the outdoor mounted condensers for the subsystems may be rejecting a considerable amount of heat to the atmosphere, while the heating requirements in the comfort conditioned spaces of the supermarket may be substantial. It has long been recognized that heat otherwise rejected to the atmosphere by the condensers can be reclaimed for use in heating the store. This is done by introducing another heat exchange coil, called the heat reclaim coil, which is usually connected in series between the compressor and the outdoor condenser, but which is placed in thermal contact with the air in the comfort conditioned space, for example in a heating duct. In a conventional heat reclaim system, a fixed head pressure control system limits the minimum condensing temperature and pressure. The minimum condensing pressure is set high enough to provide adequate subcooling for all expansion devices, to provide adequate condensing temperature for gas defrosting, and to provide adequate temperature difference between the heat reclaim coil and its inlet air.
Reclaimed heat from this type of prior art system is often thought to be "free" in that no additional energy input requirements are involved. However, this may or not be correct, depending upon the actual operating conditions. Typically, the heat reclaim coil requires a higher condensing temperature and pressure than either subcooling or defrosting considerations require, because of the design of the heat exchanger, and the fact that it is interfacing with air that is already in the 60.degree. F. to 70.degree. F. range. The higher condensing temperature and pressure causes the compressor motor power consumption and running time to increase in order to maintain the higher head pressure, and this increased energy input into the system required for heat reclaim operation must be considered when examining heat reclaim economics. For example, in prior art systems that use a fixed high head pressure and temperature for reclaim, energy is lost during the time that the environmental control thermostat in the supermarket is not calling for additional heat and the heat reclaim coil is temporarily switched out of the circuit. In that situation, the head pressure is maintained higher than required for either defrosting or for subcooling at the expansion devices and the extra energy expended by the compressor motors tends to offset the supposed savings in heat reclaim. Some prior art systems have proposed to eliminate some of this waste by switching between two fixed head pressures, a lower one when reclaim is not being used, and a higher one when it is. Although that method improves efficiency somewhat, all prior art systems are still subject to inefficiencies as follows.
In prior art systems, the condensing pressure and its corresponding temperatures for heat reclaim are usually selected in consideration of the design of the heat reclaim coil, and the heating requirements of the store. If more heat is required in a given application, it is possible to recover more heat from the reclaim coils simply by raising the system head pressure. However, it has not generally been recognized heretofore that increasing the head pressure beyond a certain point leads to a region of operation where the energy input cost for the additional heat that is reclaimed is not economically competitive with the costs of equivalent heat produced through the burning of fossil fuels.
We have determined the points of maximum efficiency of heat reclaim, and we have devised our control system to take advantage thereof.
In one aspect of the present invention, we provide a control system for a refrigeration system of the type including a condenser which is, at least at some times, subject to low ambient temperatures, and a heat reclaim coil for recovering heat into the building. A hierarchy of condensing temperatures and pressures is established, corresponding to the needs of various modes of operation including normal refrigeration, defrosting, and heat reclaiming. Sensors and control devices are provided for controlling system pressure to the minimum pressure required by the highest pressure mode in the hierarchy which is active at a given time. When neither defrost nor reclaim is required, head pressure is held to the minimum required to insure subcooling and minimum pressure differential at the expansion devices. When defrosting is required, heat pressure is boosted into an amount required for that mode of operation.
According to another aspect of the invention, when heat reclaim is required, a diverting valve is activated so that the condenser will discharge into the heat reclaim coils, which in turn will discharge into the outdoor condenser. If only a small amount of heat reclaim is required, head pressure is not boosted, but is left under the control of lower pressure modes. In this manner, some heat reclaim takes place by desuperheating in the coil.
If additional heat is required by the thermostatic control system in the building, the control system of the present invention increases the amount of heat reclaim by causing the condensing pressure, and temperature to be controlled so as to maintain the point of complete condensation at or near the output of the heat reclaim coil. In the preferred embodiment, this is accomplished by the use of a subcooling sensor positioned at the output of the heat reclaim coil, and connected to control head pressure to maintain a slight amount of subcooling at that point. It has been determined that further increasing head pressure above that point, while resulting in additional heat output from the reclaim coil, rapidly becomes as costly as supplying the additional heat requirement with electrical resistance heat. Where fossil fuel heat is available, it is preferable to bring furnaces into play when heating requirements exceed the above-mentioned optimum heat reclaim operation points.
In the case of multiple refrigeration systems, comprising a plurality of subsystems, the present invention provides a sequence control for successively bringing the individual subsystems into heat reclaim mode with increasing heating demands in the building. The diverter valves are first activated, either sequentially, or in unison to bring the heat reclaim coils for the various subsystems into the desuperheat mode. If more heat is needed, a first subsystem is brought to full condensing in the heat claim coil by placing its pressure control valve under control of the subcooling sensor for the heat reclaim coil. If additional heat is needed, the next subsystem is brought up to full condensing in the reclaim coil, and so on until all subsystems are in that condition. If additional heat is then needed, fossil fuel furnace heating units are brought into play. In case suitable fossil fuel heating units are not available, electric resistance heating, or further increased head pressure in the refrigeration systems can be utilized, but at a much higher cost, per unit of heat energy.